Television transmitting tube



Sept. 3, 1940. H. A. lAMs TELEVISION TRANSMITTING TUBE Filed May 5l,1938 bini. 1114.

2:3 A. INVENTOR.

HAI/QL E Y 4. [AMS @MQW/cz,

ATTORNEY.

Patented Sept. 3, 1940 UNITED STATES naar PATENT oEEICE TELEVISIONTRANSMITTING TUBE Delaware Application May 31, 1938, serial No. 210,952

6 Claims.

My invention relates to cathode ray tubes of the television transmittingtype and more particularly to improved television transmitting tubes ofthe lovv electron velocity type and methods of operating such tubes intransmitting systems.

In many cathode ray television transmitting tubes an optical image isprojected on a mosaic electrode which causes the elemental areas of themosaic to acquire electrostatic charges Which form what may be termed anelectrostatic image of the projected optical image and which areneutralized to produce picture signals by scanning the mosaic electrodewith a high velocity electron beam. When such high velocity scanning isused, the surface of the mosaic electrode at the point of impact of thebeam emits secondary electrons which tend to ow to those elemental areasof the mosaic which are more highly illuminated and therefore morepositive than other areas, so that the distribution of the secondaryelectrons is non-uniform and conse-k quently neutralization of thecharges on these elemental areas is unequal. The non-uniformdistribution of secondary electrons may produce a spurious signalcommonly referred to as dark spot signal, and in addition, the return ofsecondary electrons at random to the active surface of the mosaicelectrode decreases the quantity of electrons utilized in'generating thesignal With a consequent loss in e-ciency of operation. It has beenproposed to so reduce the velocity o-f the electron scanning beam thatit Will produce little or no secondary electron emission uponimpingement With the mosaic electrode by providing an auxiliary cathodeWhich is scanned by a moving spot of light to generate photoelectronswhich are projected at low velocity on the mosaic electrode. 'Iheauxiliary cathode is provided closely adjacent and parallel to themosaic electrode but while this arrangement is suitable with mosaicelectrodes of the double sided type, it is not equally advantageous withmosaic electrodes of the single sided type because the radiant energyfrom the moving spot of light on the auxiliary cathode is reflected byor transmitted through the auxiliary cathode and becomes incident uponthe mosaic electrode resulting in background illumination whichdiminishes the overall contrast of the optical image to be transmitted.

One object of my invention is to provide a television transmitting tubeof the single sided mosaic electrode type Which electrode may `bescanned Without the introduction of spurious signals such as caused bybackground illumination Or the generation of dark spot phenomena. An-

(Cl. Z50- 153) other object of my invention is to provide a tube ofgreater sensitivity capable of producing tele-- vision signals Which areof greater intensity than those obtained in the usual way and which maybe obtained without appreciable distortion. y5

In accordance with my invention, an electrostatic image, correspondingin electrostaticenergy distribution to the light distribution of anoptical image, is formed on a mosaic electrode preferably of the singlesided type Which is l0 scanned by magnetically directed loW velocityelectron streams originating at a flying spot produced Where a scanninglight beam or beam of radiant energy impinges on a scanned photoelectronemissive surface which is displaced from the l5 mosaic electrode in sucha manner that light or radiant energy from the flying spot cannot bereected or transmitted in such a manner as yto be incident upon themosaic electrode.

A better understanding of my invention Will be 20 obtained and otherobjects, features, and advantages of my invention Will appear from thefollowing description taken in connection with the accompanying drawingin which;

Figure l is a longitudinal `perspective View par- 25 tially in sectionillustrating one form of my television transmitting tube and itsattendant circuits; and,

vFigure 2 is a perspective View of a modification of the structure shownin Figure l.

Referring to the drawing and particularly to Figure 1, my televisiontransmitting tube com--l prises a highly evacuated cylindrical envelopeor .bulb I having tWo optically uniform Windows 2 and 3. Within theenvelope and opposite the 35 Window 2 is positioned a flat mosaicelectrode il of the single sided type to receive an optical image of anobject such as represented by the arroW 5 and focused on the mosaicelectrode by the lens system A photocathode 'l is positioned 40 Withinthe envelope opposite the Window 3'with its photosensitive surfaceelectronically exposed along semi-circular paths Wholly Within theenvelope to the photosensitive surface of the mosaic electrode 4 and insubstantially co-planar rela- 45 tionship With the mosaic electrode Iland also in such a position as to be scanned by a moving spot of lightgenerated in the cathode ray tube 8 and focused through the Window 3 andupon the photocathode l by the lens system 9. scanning the electronimage, which is developed on the mosaic electrode by light incidentthereon, there is between the mosaic electrode 4 and the photocathode la coarsely Woven ne Wire mesh electron permeable accelerating electrodel@ To assist in 50` which preferably lies in a plane substantiallyperpendicular to the plane of the mosaic electrode 4 and thephotocathode l. While the accelerating electrode I is preferably of themesh type, it may, however be a ring type electrode of conductingmaterial on the wall of the bulb i between the mosaic electrode 4 andthe photocathode '1. Surrounding the envelope l and preferably extendingslightly within the confines of the cylindrical bulb, such as by flaringinwardly that portion of the bulb designated as H, is an electron`focusing coil I 2, interposed substantially midway between theelectrode 4 and the photocathode l', a portion thereof` being in theplane of these electrodes, for the purpose of focusing electrons fromthe photocathode 1 to the mosaic electrode 4 Which electrons areliberated under the influence of the spot of light generated in thecathode ray tube 8 and focused on the photocathode 'i by the lens system9. The focusing coil, when energized from a source of direct current,produces a magnetic eld having lines of force which are semi-circular orsubstantially semi-circular and which intersect the mosaic E electrodeand photocathodein a direction substantially normal to their surfaces.Such a focusing field is substantially free of distortion if thefocusing coil l2 is of circular form and has an inside diameter equal toor greater than the maximum distance between the remote edges of themosaic electrode and photocathode.

It is desirable to make the fluorescent screen of the cathode ray tubeof material such as calcium tungstate which has little phosphorescenceafter being scanned by the cathode ray beam developed in the cathode raytube 8. This tube is conventional having the usual cathode and electronbeamfocusing anodes by the cooperative action of which an electron beamis directed to the fluorescent screen in the usual manner.

The mosaic electrode 4 is of the conventional single sided type andcomprises a multiplicity of mutually separated and mutually insulatedphotosensitive particles i3 to provide a discontinuous photosensitivesurface deposited on an insulating foundation, such as a sheet of micaM, provided with a conductive coating I5 preferably on the side of themica sheet it opposite that bearing the photosensitive particles. Such amosaic electrode, together with its method of manufacture, is shown byS. F. Essig in his U. S. Patent 2,065,570. As is customary in the art,the conductive coating l5 is connected to ground through the usual loadresistance i6 and to the input electrode of the translating device, suchas the triode l'l. The photocathode 'l is maintained at ground potentialand the accelerating electrode l@ is maintained at a small positivepotential, such as l to 20 volts, with respect to ground. Thephotocathode 'l is of the conventional type and comprises a foundationsuch as a sheet of mica i8 having a continuous photosensitized surfacefacing the optically uniform window 3. The

methods of forming such photosensitive surfaces are well known in theart but I prefer to form this photosensitive surface by depositing onthe mica sheet i3, a thin film of silver I9 which is oxidized andsensitized by exposure to caesium in the usual manner.

In operation, an optical image of an object, such as represented by thearrow 5, is projected and focused upon the mosaic electrode i by thelens system 6, electrons liberated from the illuminated surface of themosaic electrode being accelerated by and passing through theaccelerating electrode l0 and collected by the photocathode Because thephotosensitive particles I3 of the mosaic electrode 4 are mutuallyseparated and insulated one from another, a positive electrostatic imageis formed on the mosaic electrode by the loss of photoelectronsliberated under the influence of the optical image. The fluorescentscreen of the cathode ray tube 3 is scanned by an electron beam to forma pattern preferably consisting of horizontal lines each displaced fromthe preceding line in a direction normal to the lines. An optical imageof the pattern is projected upon the photocathode 'l by the lens system9 and forms in effect a scanning light spot on the photocathode 1. rIhevvord pattern is here used because the trace of the cathode ray beam inthe tube 8 retraces its path many times per second or at a frequencysuiicient for normal operating requirements, and appears as a patterndue to persistence of vision. The focusing coil l2 is energized from asource of direct current (not shown) and generates an electron focusingfield having electromagnetic lines of force of semi-circular form normalto the illuminated surface of the mosaic electrode 4 and to thephotocathode l. Electrons liberated from the surface of the photocathodeare accelerated by the accelerating electrode lli and directed alongsemi-circular paths coincident with the lines of magnetic force whilemaintaining their same relative positions with respect to their pointsof origin by the magnetic field generated by the coil l2 and impingewith low velocity on those areas of the mosaic electrode which havereceived a positive electrostatic charge due to the loss ofphotoelectrons under the influence of the optical image. Those electronswhich are liberated from the surface of the photocathode at the pointswhere the semi-circular electromagnetic lines of force intercept thephotosensitive particles I3 having zero or negative potentials aredecelerated and accelerated in the opposite direction by theaccelerating electrode l0 and returned to the photocathode l. In thismanner the electrostatic image representative of light and shade areasof the optical image projected upon the mosaic electrode are neutralizedby the electrons liberated from the photocathode by the moving spot oflight. Since the photosensitive particles l3- are in capacitiverelationship with the conductive coating l5 and retain an electrostaticcharge proportional to the light intensity incident thereon, each timethe electrostatic charge is neutralized by the electrons liberated fromthe photocathode under the iniiuence of the moving light spot, a signalwill be generated in the impedance I6 which isapplied to the translatingdevice Il whereupon it is further amplified and transmitted in the usualmanner.

While I have shown, in connection with Figure 1, the main features of myinvention, I have found equally advantageous an arrangement such asshown in Figure 2. As best shown in Figure 2, the electrons liberatedfrom the surface of the photocathode 'i are directed along semicircularpaths by a magnetic field developed by thel horseshoe type magnet Elihaving two pole faces 2l and 22 one of which is closely adjacent,coextensive with, and parallel to the mosaic electrode Ll, the other ofwhich is similarly positioned with respect to the photocathode l. Thatportion of the cylindrical envelope adjacent the rear surfaces -of themosaic electrode and photocathoder are-preferablylflattened to allowcloser posi- Cil CII

tioning of the pole`face`s 2l and 22. All of the other structure shownin Figure 2 is similar to, and is similarly referenced as, that shown inFigure l. The horseshoe type magnet 20 is energized by a coil 23 throughwhich there is caused to flow a direct current of sufficient magnitudeto generate the required electromagnetic field which extends assemi-circular lines of force from the surface of the mosaic electrode Llto the surface of the photocathode 7. I have found that a field strengthof approximately 100 gausses is sufficient to obtain proper focusingofthe low velocity electrons which flow between the mosaic electrode andphotocathode and vice versa. With such a eld strength, the electronsfrom points on the photocathode are directed to correspondingly spacedpoints on the mosaic electrode.

It will be evident from the foregoing description of my new and improveddevice that the electrostatic image generated on the mosaic electrode,and which corresponds in intensity to the light intensity of the opticalimage, is neutralized by low velocity electrons liberated from thephotocathode and directed upon the mosaic electrode by the magneticfocusing field which electrons are incapable of generating secondaryelectrons on the surface of the mosaic electrode which might benon-uniformly distributed thereby causing spurious signals such as darkspot signals. It is likewise evident that, since the optical axis of thelens system 6 is displaced from and parallel to the lens system 9, therecan be no interference between the light of the optical image and thelight generated on the fluorescent screen of the cathode ray tube 8.Furthermore, since the mosaic electrode 4 and the photocathode 'l are inthe same plane and displaced one from the other, it isvirtuallyimpossible for the light of the ying light spot to be reflectedor transmitted in such a manner as to become incident upon the mosaicelectrode. With this arrangement of the electrode structure, manydifficulties with respect to the transmission of back- .ground lightingeffects are obviated, resulting in a television transmission which isrelatively free of spurious signals and background disturbances.

From the foregoing description, it will be apparent that various othermodifications may be made in my invention without departing from thespirit and scope thereof and I desire, therefore, that only suchlimitations shall be placed thereon as are necessitated by the prior artand as set forth in the appended claims.

What I claim is:

1. A television transmitting device including in an evacuated envelopetwo electron emitting elec'- trodes, one of which is of the mosaic typeand comprises a foundation of electrically insulating material, a mosaicof mutually separated and insulated photosensitive particles on one sideof said foundation and a conductive coating in capacitive relation withsaid particles, both of said electrodes being in substantially the sameplane, each displaced one from the other and electronically exposed oneto the other along semi-circular paths to allow an unimpeded electron owtherebetween, means without said envelope and facing the side of saidphotocathode exposed to said electrode of the mosaic type for projectinga moving spot of light on said photocathode, means without said envelopeand facing the side of said electrode of mosaic type exposed to saidphotocathode to project an optical image on said electrode of mosaictype, and a magnetic focusing coil surrounding the envelope 'and lyingin a planesubstantially perpendicular to said photocathode and 4saidelectrode of mosaic type and substantially midway between the adjacentedges of said electrodes to generate a curved magnetic eld which extendsbetween said electrodes and intersects the 'surfaces of said electrodesto focus electrons from each of said electron emitting electrodes to theother of said electrodes.

2. A television transmitting device comprising evacuated envelope, twophotosensitive elec*- trodes, one of which is of the mosaic type withinsaid envelope positioned in a single plane and displaced one from theother, the photosensitive surfaces of said electrodes facing and beingelectronically exposed to one another along semicircular curved paths,means to project light representative of an optical image from the sideof and upon the surface of the electrode of mosaic type facing andelectronically exposed to said other electrode, means to project amoving spot of light from the side ofand upon the corresponding surfaceof said other electrode, and a magnetic focusing coil having a portionthereof in the plane of said electrodes and surrounding said envelope ata section of said envelope between said electrodes, said coil beingdisplaced from the optical axes along which light representative of saidoptical image and said light spot are projected.

3. A television transmitting device comprising an evacuated envelope,two photosensitive electrodes within said envelope, side by side and insubstantially a single plane, one of said electrodes having a continuousphotosensitive surface and the other a discontinuo-us photosensitivesurface, said electrodes each having their photosensitive surfacesfacing one another along semi-circular paths wholly within saidenvelope, means without said envelope to focus electrons from oneelectrode to the other comprising a magnetic coil lying in a planesubstantially perpendicular to and midway between said electrodes togenerate a magnetic eld having lines of force of substantiallysemi-circular form extending between and intersecting the surface ofeach electrode in a direction substantially normal to the surface ofeach electrode, means facing the discontinuous photosensitive surface ofone of said electrodes to project light representative of an opticalimage along an optical axis parallel to the plane of said coil and onthe said discontinuous surface, and means facing the continuousphotosensitive surface of the other of said electrodes to project amoving spot of light on said other surface.

4. A television transmitting device having an evacuated envelope, aphotosensitive mosaic electrode, and a photocathode in coplanar relationwithin said envelope, the photosensitive surfaces of said electrode andsaid photocathode facing one another along semi-circular paths Whollywithin said envelope, a substantially circular magnetic focusing coilbetween adjacent edges of said mosaic electrode and said photocathodeand surrounding said envelope, said coil having an inside diameter atleast equal to the maximum distance between the remote edges of saidmosaic electrode and said photocathode, means facing the photosensitivesurfaces of said mosaic electrode to project light representative of anoptical image directly upon the photosensitive moving spot of light as ascanning pattern on the photosensitive surface of said photocathode-togenerate a beam of electrons for neutralizing said electrostatic imageand generating signals representative of said optical image.

5. A television transmitting device as claimed in claim 4 having inaddition an electron permemosaic electrode and a photocathode, eachopposite one of said window portions and in coplanar relation one withthe other with `their photosensitive surfaces facing said windowportions and electronically exposed one to the other along semi-circularpaths within said envelope, an electron permeable accelerating electrodebetween said window portions and between and substantially perpendicularto said mosaic electrode and said photocathode, and means without saiden- Velope for focusing streams of electrons from said photocathode uponsaid mosaic electrode, said means comprising a horseshoe magnet havingtwo pole faces, one of which is closely adjacent, coextensive with, andparallel to the surface of said mosaic electrode opposite thephotosensitive surface thereof facing one of said window portions, theother pole face of which is similarly positioned with respect to saidphotocathode to generate a magnetic field having lines of forceextending between and intersecting the photosensitive surfaces of saidmosaic electrode and said photocathode.

HARLEY A. IAMS.

